Automatic reversing circuit for a window regulator motor control system

ABSTRACT

A window regulator motor control system for a drive motor operative to open and close a window includes an automatic reversing circuit incorporating an overload current relay and a reversing relay which function to reverse the motor in response to an overload current condition. The reversing relay is energized through a controlled rectifier gated conductive by a triggering circuit including a resistance-capacitance time delay network connected to the overload current relay. When the window is driven against an obstruction, the time delay network provides a momentary delay before the controlled rectifier is gated conductive and the motor is reversed.

United States Patent Inventor Dale F. Hammer Bellbrook, Ohio Appl. No.870,628

Filed Dec. 15, 1969 Patented May 25, 1971 Assignee General MotorsCorporation Detroit, Mich.

AUTOMATIC REVERSING CIRCUIT FOR A WINDOW REGULATOR MOTOR CONTROL SYSTEMPrimary Examiner-Gris L. Rader Assistant Examiner-K. L. CrossonAttorneys-E. W. Christen, C. R. Meland and Robert W.

Smith ABSTRACT: A window regulator motor control system for a drivemotor operative to open and close a window includes an automaticreversing circuit incorporating an overload current relay and areversing relay which function to reverse the motor in response to anoverload current condition. The reversing relay is energized through acontrolled rectifier gated conductive by a triggering circuit includinga resistancecapacitance time delay network connected to the overloadcurrent relay. When the window is driven against an obstruction, thetime delay network provides a momentary delay before the controlledrectifier is gated conductive and the motor is reversed.

PATENTED mam 3581.174

I N VISN'I OR.

fizz/e 9. fiizmmer ATTORNEY AUTOMATIC REVERSING CIRCUIT FOR A WINDOWREGULATOR MOTOR CONTROL SYSTEM This invention relates to motor drivenwindow regulators and more particularly to a motor control systemincluding a reversing circuit responsive to an obstacle impedingmovement of the window regulator.

The use of remotely controlled and power assisted devices are nowcommonplace in automotive vehicles and in particular electricallypowered window regulators are more frequently found among the accessoryitems included in automobiles. Operation of remotely controlledautomobile windows can include instances of a window engaging anobstruction without the knowledge of the operator. This may occur, forexample, when an occupant in a forward compartment closes a window in arearward compartment. ln the case of a window being driven against anobstructing object for an extended time, possible damage can occur tothe window regulator mechanism or to the motor as a result of increasedmotor current which develops because rotation of the motor is blocked.

In accordance with the present invention, a remotely controlled windowis automatically retracted in response to an obstruction impedingclosure of the window by incorporating an improved automatic reversingcircuit in the window regulator motor control system. More specifically,an overload current relay anda reversing relay are provided to switchthe motor so as to open the window in response to an overload currentcondition. The reversing relay includes relay contacts connected to themotor so as to bypass the normal motor power circuit. A semiconductorcontrolled rectifier energized the reversing relay and is gatedconductive by a triggering circuit connected to switch contacts of theoverload relay. The triggering circuit includes a resistance-capacitancetime delay network having a capacitor which begins charging when theswitch contacts close in response to motor overload current developedwhen the window is being closed. Voltage developed across the capacitorrenders the controlled rectifier conductive after a momentary'time delayas determined. by the time constant of the time delay network. Thecontrolled rectifier energizes the reversing relay and the relaycontacts close to interrupt the motor current and reconnect the motorfor reverse rotation. The window is driven open to free the obstructionimposed on the operation of the window regulator and to eliminate theoverload current condition.

Accordingly, an object of the present invention is to provide animproved automatic reversing circuit arrangement for a window regulatordrive motor in which simple and reliable circuit components are easilyincorporated in a window regulator motor control system so as to reversethe rotation of the drive motor in response to an overload currentcondition.

A further object of this invention is to provide an automatic reversingcircuit for a window regulator drive motor having two field windingswherein an overload relay senses the current in one of the fieldwindings and a series circuit including a controlled rectifier energizesa relay coil of a reversing relay having contacts operative to interruptthe current in the one field windings and to energize the other fieldwinding and further, a triggering circuit connected between switchcontacts of the overload current relay and the controlled rectifiergates the latter conductive so as to energize the reversing relay aftera predetermined delay period thereby avoiding reversing of the motor inresponse to instantaneous surges of high current.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing wherein a preferred embodiment of the present invention isshown.

FIG. 1 is a partial side elevational view of an automotive vehicle bodyillustrating a window regulator actuated by a window regulator motorcontrol system including the automatic reversing circuit of thisinvention.

FIG. 2 is a schematic electrical diagram of the window regulator motorcontrol system and the automatic reversing circuit of this invention.

Referring to P10. 1 of the drawing, there is shown an automotive vehiclebody 8 with a window opening 10 including a window closure memberprovided by a movable window 12 which is retractable into a hollow bodysection 14. A window regulator 16 mounted in the body section 14 movesthe window 12 between the closed and open positions. An electric drivemotor 18 powers the window regulator 16 in accordance with operation ofa window regulator motor control system 20. A manually operated switch22, accessible to an occupant of the vehicle body 8, provides remotelycontrolled operation of the motor control system 20 as describedhereinbelow in connection with description of F IG. 2.

The window regulator 16 includes a linkage mechanism corresponding tothat disclosed in U.S. Pat. No. 3,069,152, issued Dec. l8, i962 andassigned to the assignee of this invention, which is illustrative of asuitable window regulator mechanism adapted to be powered by motor drivemeans such as the motor 18. The linkage mechanism generally includes apair of pivot arms 24 and 26 having upper ends connected to the window12 by a slotted channel 28. The motor 18 is drivingly connected to thewindow regulator 16. through a gear arrangement including a pinion gear30 and a sector gear 32 fixedly mounted on the arm 26. The motor 18 ismounted on the lower end of the arm 24 which pivots about a fixed pivotpoint 36. The arm 26 has a lower end slidably mounted to the vehiclebody by a cam channel 38.

The motor 18. is a series type direct current motor including a firstfield winding 40, a second field winding 42, and an armature 44. Themotor circuit includes the field windings and 42 connected in separateseries circuits with the armature v 44 which is externally connected toa source of reference potential at a grounded connection 45. A rotatablemotor shaft 46 associated with the armature 44 is operatively connectedto the pinion gear 30. The field windings 40 and 42 are selectivelyenergizable to rotate the motor shaft 46 clockwise and counterclockwiseso at to raise and lower, respectively, the window 12.

THe motor control system 20 includes a motor power circuit 50 and anautomatic reversing circuit 52 provided by the present invention. Thepower circuit 50 includes the manually operated switch 22 connectedbetween the motor 18 and a source of direct current voltage provided bya battery 54. The automatic reversing circuit 52 includes reversingrelay 56 and an overload current relay 58 respectively including areversing relay contact assembly 60 and a current relay winding 62, bothconnected in the power circuit 50 as described hereinbelow.

THe battery 54 has the positive pole thereof connected through anignition switch 66 to a circuit junction 68 to supply both the powercircuit 50 and the automatic reversing circuit 52. The negative pole ofthe battery 54 is connected to the reference potential at a groundedconnection 70.

The power circuit 50 is connected to the junction 68 by the contactassembly 60 of the reversing relay 56. The contact assembly 60 includesa movable contact arm 72, a normally closed contact 74 and a normallyopen contact 76. The arm 72 includes common contact 77 which engages thenormally closed contact 74 when a relay coil 78 included in thereversing relay 56 is deenergized. The contact arm 72 is connected tothe circuit junction 68 and is operated by an armature 80 movable inresponse to energization of the relay coil 78 to disengage the contact74 and engage the contact 77 with the contact 76.

The manually operated switch 22 is connected between the norlnallyclosed contact 74 and the motor 18 to selectively energize the fieldwindings 40 and 42. A switch lever 82 is movable between two positionsto engage contacts 84 and 86. The contact 84 is designated Close and isconnected with the field winding 40 and the contact 86 is designatedOpen and is connected with the field winding 42. The switch lever 82 isnormally biased intermediate the contacts 84 and 86 so that the motor isnot energized until the switch lever 82 contacts either of the contacts84 or 86.

The winding 62 provided in the overload current relay 58 is connectedintermediate the contact 84 and the field winding 40. The winding 62 iscurrent sensitive and is responsive to excessive or high current levelsconducted through the field winding 40. The switch contact assembly 88of the overload current relay 58 includes normally open contacts 90 and92 of a reed switch inductively coupled with the winding 62. In apreferred embodiment the winding 62 is wound around the reed switch sothat the magnetic field developed by the winding 62 draws the contacts90 and 92 together when the current through the winding 62 reachesapredetermined level.

The field winding 42 is connected by a conductor 93 to the contact 86 ofthe manually operated switch 22. The field winding 42 is also connectedto the normally open contact 76 of the relay contact assembly 60 by aconductor 94 at a circuit junction 96 formed with the conductor 93. Thecontact 76 and the conductor 94 provide a path which bypasses themanually operated switch 22 so that the field winding 42 is energizeddirectly from the battery 54 when the contact arm 72 engages the contact76.

The motor power circuit 50 described hereinabove is rendered operativewhen the ignition switch 66 is closed to connect the switch lever 82 ofthe switch 22 through the normally closed contact 74 of the relaycontact assembly 60. The manually operated switch 22 is then operable toapply battery voltage across either of the field windings 40 or 42 andthe armature 54 when the switch lever 82 is placed in the Close or Openpositions.

Referring now to the automatic reversing circuit 52, the reversing relay56 is energized through a series circuit comprising conductor 102connected to the junction 68, a limit switch 104, and a controlledrectifier 106 connected between limit switch 104 and the reversing relaycoil 78. The limit switch 104 includes a stationary contact 108 and amovable contact member 109. The contact member 109 engages thestationary contact 108 and includes conductive contact surface 110located between two nonconductive contact surfaces 111 and 112. Thecontact member 109 is mounted in a suitable location for movement withthe'window 12, for example, on the sector gear 32. Therefore, theconductive surface 110 and nonconductive surfaces 111 and 112 are movedrelative to the stationary contact 108 as the window regulator 16operates between raised and lowered window positions. The nonconductivesurfaces 111 and 112 engage the contact 108 when the window is withinapproximately one-half inch of the raised and lowered positions,respectively.

The controlled rectifier 106 is a semiconductor switching device of thesilicon controlled rectifier (SCR) type having anode 114, cathode 116,and gate 118 electrodes. The anode 114 is connected to the movablecontact member 109 of the limit switch 104. The cathode 116 is connectedto the upper end 119 of the relay coil 78. The series circuit of therelay coil 78 is completed by connecting the lower end 120 of the relaycoil 78 to a grounded connection 121. The gate 118 of the controlledrectifier 106 is connected to a triggering circuit 122 operative to biasthe controlled rectifier 106 conductive.

The triggering circuit 122 includes a current limiting resistor 124 anda time delay network formed by a resistor 128 and a resistor 130connected in parallel with a capacitor 132. The resistor 124 isconnected between the gate 118 and a circuit junction 136 which isformed between one of common ends of the resistor 130 and the capacitor132 and one end of the resistor 128. The other of the common ends of theresistor 130 and the capacitor 132 is connected to the referencepotential provided at a grounded connection 138. The triggering circuit122 is completed by connecting the remaining end of the resistor 128 tocontact 92 of the switch contact assembly 88 and connecting the contact90 to a circuit junction 142 formed with the conductor 102. Thusconnected, the triggering circuit 122 is connected across the battery 54when the contacts 90 and 92 are closed by the relay winding 62. When thepotential at the circuit junction 136 applies the critical triggeringvoltage across the gate 118 and the cathode 116,

the controlled rectifier 106 is gated conductive in accordance with thewell known characteristics of a semiconductor controlled rectifier.

The time delay network provided by the resistor 128 and the parallelconnected resistor 130 and the capacitor 132 has a predeterminedresistance-capacitance (RC) time constant to keep the gating of thecontrolled rectifier 106 insensitive to an instantaneous closing of thecontacts and 92. This may occur, for example, when the field winding 40is initially energized and an instantaneous surge of starting currentoccurs. The surge of starting current is sufficient to cause the relaywinding 62 to momentarily close the contacts 90 and 92. It has beenfound that a surge of starting current exists for approximately 0.lsecond. Accordingly, the RC time constant of the time delay network hasa predetermined value such that 0.2 second is required before thecapacitor 132 is charged sufficiently to develop a voltage at thejunction 136 to render the controlled rectifier 106 conductive. Thus, ahigh current level must flow through the relay winding 62 for at least0.2 second in order to render the controlled rectifier 106 conductiveand energize the reversing relay coil 78.

During normal operation of the window actuator 16, the switch lever 82is moved to the Close position to energize the field winding 40 from theseries circuit including the battery 54, the ignition switch 66, thenormally closed contact 74, the contact 84 and the relay winding 62. Themotor shaft 46 is rotated clockwise to drive the window regulator 16 toa window raised or closed position. To lower the window 12, the switchlever 82 is placed in the Open position to engage the contact 86 andenergize the field winding 42 as described for the field winding 40.

During the window raising mode of operation, an object in the windowopening 10 can block further closing of the window 12. The currentsupplied to the field winding 40 increases because the motor shaft 46 isstopped since the torque on the shaft exceeds the output-of the motor18. As the current through the first field winding 40 begins to reachthe overload or stalled condition, the current through the relay coil 62develops a magnetic field sufficient to close the contacts 90 and 92.The-time delay network is then connected across the battery 54 and thecapacitor 132 charges in accordance with the aforementioned RC timeconstant provided by the resistances of resistors 128 and 130 and thecapacitance of the capacitor 132. The voltage at the circuit junction136 rises so that the critical firing voltage of the controlledrectifier 106 is reached after the predetermined time delay of 0.2second. The current limiting resistor 124 limits the current applied tothe gate 118 to a safe value so as to prevent damage to the controlledrectifier 106.

The controlled rectifier 106 in the conductive state provides a lowresistance path between the anode 114 and cathode 116. Accordingly, thereversing relay winding 78 is connected across the battery 54. The limitswitch 104 will be positioned so that the conductive surface engagesstationary contact 108 and provides a closed circuit path. Uponenergization of the reversing relay coil 78, the contact arm 72 is movedby the armature 80 to engage the normally open contact 76. Thisinterrupts the power circuit energizing the field winding 40 and theoverload current condition in the motor 18 is removed. Theelectromagnetic field of the relay coil 62 collapses and the associatedswitch contacts 90 and 92 are opened. Thus, the triggering circuit 122is disconnected from the battery 54 although the controlled rectifier106 continues conducting in accordance with the conventionalcharacteristics of a controlled rectifier. Voltage stored on thecapacitor 132 will drain off through the resistor to ground, dischargingthe capacitor 132.

Upon closing of the contact arm 72 with the normally open contact 76,the field winding 42 is connected to the battery 54. The conductor 94extending between the normally open contact 76 and the circuit junction94 bypasses the manually operated switch 22. Therefore, regardless ofthe position of the switch lever 82 and field winding 42 continues to beenergized.

Energization of the field winding 42 causes the armature 44 to berotated counterclockwise driving the window regulator 16 to lower thewindow 12 toward its opened position. Continued lowering of theregulator 16 rotates the conductive surface l of the limit switch 104until the nonconductive portion 112 engages the fixed contact 108. Theseries circuit including the controlled rectifier 106 and the reversingrelay winding 78 is interrupted to render the controlled rectifier 106nonconductive and deenergize the relay coil 78. The contact arm 72 isreleased and returns to engage the normally closed contact 74. Thecontrolled rectifier 106 is switched nonconductive by interrupting thevoltage across the anode to cathode and it remains nonconductive untilthe anode to cathode voltage is reapplied and a triggering voltage isapplied to the gate 118.

If the manually operated switch lever 82 has been released, the window12 will remain in the opened position. If the switch lever 82 is placedin the Close position, the circuit for energizing the first fieldwinding 40 is again operable to drive the window 12 toward the closedposition. As the window closes, the limit switch 104 is rotated so thatthe nonconductive surface 111 engages the fixed contact 108. Thus, theautomatic reversing circuit 52 of this invention is disconnected andinoperative when the window 12 is in either the opened or closedposition. Any overload current that may be developed by continuing toenergize the windings 40 and 42 from the switch 22 does not activate thereversing circuit after the closing and opening operations arecompleted.

From the foregoing description it is apparent that the inventionprovides a simple and effective means for preventing damage to the motor18 or the window regulator 16 caused by obstruction preventing themovement of the window 12. The automatic reversal of the motor 18prevents motor current from reaching an overload level for an extendedtime after the motor is stopped by an obstruction. The circuit elementscomprising the automatic reversing circuit 52 may be easily andcompactly mounted within a hollow body portion of an automotive vehicleas found in a door for example.

While the embodiment of the present invention as herein disclosedconstitutes a preferred from it is understood that other forms may beadopted within the scope of this invention.

What I claim is as follows:

1. A motor control system for regulating the operation-of a windowclosure member in combination with an electrical power source; a motorincluding an armature, a first field winding for rotating said armaturein one direction when energized by said electrical power source, and asecond field winding for rotating said armature in the oppositedirection when energized by said electrical power source; and a windowregulator mechanism connected to said armature and to said windowclosure member for moving the window closure member to a closed positionin response to rotation of said armature in said one direction andmoving the window closure member to an open position in response to therotation of said armature in said opposite direction; wherein said motorcontrol system comprises: a motor power circuit including at least oneelectrical switch operable to a first position for connecting saidarmature and said first field winding across said electrical powersource and operable to a second position for connecting said armatureand said second field winding across said electrical power source; areversing relay including a relay coil and a contact assembly connectedin said motor power circuit for connecting said electrical switch andsaid motor across said electrical power source while said relay coil isdeenergized and for connecting only said armature and said second fieldwinding across said electrical power source when said relay coil isenergized; a reversing circuit connected between said electrical powersource and said relay coil including a controlled rectifier havingtwocurrent carrying electrodes connected in series with said relay coil andsaid electrical power source, and a gate electrode for energizing saidrelay coil in response to the application of a trigger signal to saidgate elecoverload current relay including a relay winding and a normallyopen switch assembly; means connecting said relay winding in said motorpower circuit in a current sensing relationship with said first fieldwinding so that said normally open switch assembly is closed in responseto an overload current developed in said first field winding; meansconnecting said normally open switch assembly and said triggeringcircuit across said electrical power source whereby said triggeringcircuit is energized when said normally open switch assembly is closed;and a time delay network included in said triggering circuit fordelaying the application of a trigger signal to said gate electrode fora predetermined time period after said triggering circuit is energizedwhereby said reversing circuit is insensitive to instantaneouslydeveloped high current surges through said first field winding.

2. A motor control system for regulating the operation of a windowclosure member in combination with an electrical power source; 'a motorincluding an armature, a first field winding for rotating said armaturein one direction when energized by said electrical power source, and asecond field winding for rotating said armature in the oppositedirection when energized by said electrical power source; and a windowregulator mechanism connected to said armature and to said windowclosure member for moving the window closure member to a closed positionin response to rotation of said annature in said one direction andmoving the window closure member to an open position in response to therotation of said armature in said opposite direction; wherein said motorcontrol system comprises: a motor power circuit including at least oneelectrical switch operable to a first position for connecting saidarmature and said first field winding across said electrical powersource and operable to a second position for connecting said armatureand said second field winding across said electrical power source; areversing relay including a relay coil and a contact assembly connectedin said motor power circuit for connecting said electrical switch andsaid motor across said trode; a triggering circuit connected to saidgate electrode of said controlled rectifier for applying a triggersignal to said gate electrode when the triggering circuit is energized;an

electrical power source while said relay coil is deenergized and forconnecting only said armature and said second field winding across saidelectrical power source when said relay coil is energized; a reversingcircuit including a limit switch having a fixed contact and a movablecontact operatively coupled to movement of said window closure member, acontrolled rectifier having two current carrying electrodes connected inseries with said fixed contact and said movable contact, and connectingsaid-reversing relay coil across said electrical power source, thecontacts of said limit switch having an open contact position inresponse to said window closure member being in one of the open orclosed positions and a closed contact position over intermediatepositions between said open or closed positions, said controlledrectifier further having a gate electrode for energizing said relay coilwhen said limit switch is in the closed position in response to theapplication of a trigger signal to said gate electrode; a triggeringcircuit connected to said gate electrode of said controlled rectifierfor applying a trigger signal to said gate electrode when the triggeringcircuit is energized; an overload current relay including a relaywinding and a normally open switch assembly; means connecting said relaywinding in said motor power circuit in a current sensing relationshipwith said first field winding so that said normally open switch assemblyis closed in response to an overload current developed in said firstfield winding; means connecting said normally open switch assembly andsaid triggering circuit across said electrical power source whereby saidtriggering circuit is energized when said normally open switch assemblyis closed; and a time delay network connected in said trigger circuitincluding a first resistor, a second resistor connected in series withsaid first resistor and a capacitor connected across said secondresistor, said time delay network having a predetermined time 'constantfor delaying the application of a trigger signal to said gate electrodefor a predetermined time periodafter the triggering circuit is energizedwhereby said reversing circuit is insensitiveto instantaneouslydeveloped high current surges occurring in said first field winding.

1. A motor control system for regulating the operation of a windowclosure member in combination with an electrical power source; a motorincluding an armature, a first field winding for rotating said armaturein one direction when energized by said electrical power source, and asecond field winding for rotating said armature in the oppositedirection when energized by said electrical power source; and a windowregulator mechanism connected to said armature and to said windowclosure member for moving the window closure member to a closed positionin response to rotation of said armature in said one direction andmoving the window closure member to an open position in response to therotation of said armature in said opposite direction; wherein said motorcontrol system comprises: a motor power circuit including at least oneelectrical switch operable to a first position for connecting saidarmature and said first field winding across said electrical powersource and operable to a second position for connecting said armatureand said second field winding across said electrical power source; areversing relay including a relay coil and a contact assembly connectedin said motor power circuit for connecting said electrical switch andsaid motor across said electrical power source while said relay coil isdeenergized and for connecting only said armature and said second fieldwinding across said electrical power source when said relay coil isenergized; a reversing circuit connected between said electrical powersource and said relay coil including a controlled rectifier having twocurrent carrying electrodes connected in series with said relay coil andsaid electrical power source, and a gate electrode for energizing saidrelay coil in response to the application of a trigger signal to saidgate electrode; a triggering circuit connected to said gate electrode ofsaid controlled rectifier for applying a trigger signal to said gateelectrode when the triggering circuit is energized; an overload currentrelay including a relay winding and a normally open switch assembly;means connecting said relay winding in said motor power circuit in acurrent sensing relationship with said first field winding so that saidnormally open switch assembly is closed in response to an overloadcurrent developed in said first field winding; means connecting saidnormally open switch assembly and said triggering circuit across saidelectrical power source whereby said triggering circuit is energizedwhen said normally open switch assembly is closed; and a time delaynetwork included in said triggering circuit for delaying the applicationof a trigger signal to said gate electrode for a predetermined timeperiod after said triggering circuit is energized whereby said reversingcircuit is insensitive to instantaneously developed high current surgesthrough said first field winding.
 2. A motor control system forregulating the operation of a window closure member in combination withan electrical power source; a motor including an armature, a first fieldwinding for rotating said armature in one direction when energized bysaid electrical power source, and a second field winding for rotatingsaid armature in the opposite direction when energized by saidelectrical power source; and a window regulator mechanism connected tosaid armature and to said window closure member for moving the windowclosure member to a closed position in response to rotation of saidarmatUre in said one direction and moving the window closure member toan open position in response to the rotation of said armature in saidopposite direction; wherein said motor control system comprises: a motorpower circuit including at least one electrical switch operable to afirst position for connecting said armature and said first field windingacross said electrical power source and operable to a second positionfor connecting said armature and said second field winding across saidelectrical power source; a reversing relay including a relay coil and acontact assembly connected in said motor power circuit for connectingsaid electrical switch and said motor across said electrical powersource while said relay coil is deenergized and for connecting only saidarmature and said second field winding across said electrical powersource when said relay coil is energized; a reversing circuit includinga limit switch having a fixed contact and a movable contact operativelycoupled to movement of said window closure member, a controlledrectifier having two current carrying electrodes connected in serieswith said fixed contact and said movable contact, and connecting saidreversing relay coil across said electrical power source, the contactsof said limit switch having an open contact position in response to saidwindow closure member being in one of the open or closed positions and aclosed contact position over intermediate positions between said open orclosed positions, said controlled rectifier further having a gateelectrode for energizing said relay coil when said limit switch is inthe closed position in response to the application of a trigger signalto said gate electrode; a triggering circuit connected to said gateelectrode of said controlled rectifier for applying a trigger signal tosaid gate electrode when the triggering circuit is energized; anoverload current relay including a relay winding and a normally openswitch assembly; means connecting said relay winding in said motor powercircuit in a current sensing relationship with said first field windingso that said normally open switch assembly is closed in response to anoverload current developed in said first field winding; means connectingsaid normally open switch assembly and said triggering circuit acrosssaid electrical power source whereby said triggering circuit isenergized when said normally open switch assembly is closed; and a timedelay network connected in said trigger circuit including a firstresistor, a second resistor connected in series with said first resistorand a capacitor connected across said second resistor, said time delaynetwork having a predetermined time constant for delaying theapplication of a trigger signal to said gate electrode for apredetermined time period after the triggering circuit is energizedwhereby said reversing circuit is insensitive to instantaneouslydeveloped high current surges occurring in said first field winding.